Ion trap mass spectrometers of the type considered here contain a quadrupole ion trap operated with RF voltage. This ion trap was invented by Paul and Steinwedel, and is described in U.S. Pat. No. 2,939,952. It consists of two opposed end cap electrodes and a ring electrode situated between them in the center plane; in its theoretical ideal form, it has hyperboloids of revolution for the end cap and ring electrodes, the hyperbolas having asymptotes which intersect each other with an angle 2=arc tang (1/{square root over (2)}).
Due to the patents U.S. Pat. No. 3,527,939 (Dawson and Whetten: xe2x80x9cMass-selective storagexe2x80x9d), U.S. Pat. No. 4,548,884 (Stafford, Kelley, Stephens: xe2x80x9cMass-selective instability of ionsxe2x80x9d), US Re 34,000 (Syka, Louris, Kelley, Stafford, Reynolds: xe2x80x9cMass-selective resonance ejectionxe2x80x9d), EP 0 383 961 (Franzen, Gabling, Heinen, Weiss: xe2x80x9cMass-selective ejection by non-linear resonancexe2x80x9d) and GB 2 278 232 or DE 43 16 738 (Franzen: xe2x80x9cMass-selective ejection by superimposition of additional dipole and quadrupole alternating fieldsxe2x80x9d), methods have become known for operating ion traps as mass spectrometers which work with various kinds of mechanism for ejection of the ions one after the other (xe2x80x9cmass-sequentialxe2x80x9d), separated according to their mass-to-charge ratio (xe2x80x9cmass-selectivexe2x80x9d) and which measure the ions by means of a detector fitted outside the ion trap, generally with a secondary-electron multiplier. In these methods, the end cap electrode facing the detector is perforated so that the ions can be ejected.
Mass spectrometry cannot determine the mass of ionsxe2x80x94but only their mass-to-charge ratio, which is termed xe2x80x9cspecific massxe2x80x9d in several places in the Paul and Steinwedel patent. The ions predominantly carry only a small number of elementary charges (mostly only one). In the following, when reference is made to the mass of ions and to xe2x80x9cheavy ionsxe2x80x9d as opposed to xe2x80x9clight ionsxe2x80x9d or xe2x80x9cmass-selective ejectionxe2x80x9d, this must always be understood to mean this xe2x80x9ccharge-related massxe2x80x9d or xe2x80x9cspecific massxe2x80x9d.
The resonance methods are based on the fact that the ions can be excited in the ion trap between the end cap electrodes so that they oscillate (xe2x80x9cfundamental oscillationsxe2x80x9d or xe2x80x9csecular oscillationsxe2x80x9d). The frequency of the oscillations is strictly dependent on their charge-related mass, and also the type and strength of field in the ion trap, that is the RF voltage, the RF frequency or possibly superimposed DC voltage. For a field of constant frequency and without superimposed DC voltage, only the RF voltage and the specific mass determine the frequency of oscillation of an ion.
Due to the patents EP 0 321 819 (Franzen, Gabling, Heinen, Weiss: xe2x80x9cDistorted quadrupole field with Q less than 3.99xe2x80x9d) and EP 0 459 602 (Franzen: xe2x80x9cClean superimposition of hexapole and octopole fieldxe2x80x9d), improved forms of ion trap electrodes have become known, which produce an improvement in the ejection behavior of the ions during the scanning due to non-linear resonances by superimposition of higher-order multipole fields.
The perforations in the apex of the dome-shaped end cap electrode for the outlet of the ions represent a disturbance of the electrical field in the ion trap. This field is required to be essentially a quadrupole field, but on which higher-order multipole fields can be superimposed in a targeted manner so as to improve the oscillatory behavior of the ions in the ion trap and the enlargement of their oscillation amplitude for ejection to permit spectrum measurement. The disturbances of the field in the ion trap caused by the holes in the end cap mean that not all of the ions of a particular mass emerge at the same time: the ion signal of the ions of a particular mass is spread temporally, and the mass resolution of the mass spectrometer is reduced.
The outlet holes for the ions are usually formed as a seven-hole arrangement, and also as a single central hole in the apex of the dome-shaped end cap electrode. To minimize the disturbances of the electrical RF field in the ion trap, the holes only have a very small diameter. However, this means that not all the ions can emerge and reach the detector: more than half the ions impact on the margins of the electrodes around the holes and are discharged there. As a result, the sensitivity of the ion trap mass spectrometer is reduced.
It is known that the filling of the ion trap with ions must be restricted, as otherwise the resolving power, and also the mass calibration, that is the relationship between ejection time and exact ion mass, will be disturbed by space-charge effects. For this reason there is always an upper limit for filling the ion trap with ions; the sensitivity of the ion trap mass spectrometer therefore depends on the degree to which the limited number of ions in the trap is exploited for measurement of the spectrum.
Mass resolving power and sensitivity are, however, the essential selling criteria for ion trap mass spectrometers. A good mass resolving power permits faster scanning and thereby increases the sensitivity by increasing the number of spectra per unit of time. The sensitivity of the mass spectrometer is in demand, especially in the modem biosciences, where exceptionally small quantities of substance have to be measured. As a rule of thumb, it can be said that a possible doubling of sensitivity justifies the development of a new spectrometer.
The above-mentioned patent U.S. Pat. No. 2,939,952 of Paul and Steinwedel already proposed measuring the ion stream of the ions impacting on the end cap by measuring the effective load of the RF generator. It is now the basic idea of the invention to no longer allow the ions to emerge from the trap through holes in an end cap electrode for the purpose of detection, but to measure them when the ions impinge on the closed end cap electrode, as in Paul""s patent, but not as the effective load of the RF generator. The new idea is to integrate an ion detector into the electrode form. This ion detector exactly follows the ideal form of the end cap electrode, carries the potential of the end cap electrode on its surface and thereby causes no disturbance to the field in the ion trap. The detector can be a simple Faraday detector, which feeds the ion current to a measuring amplifier; and in particular, the ion detector can be a secondary electron multiplier.
Modern secondary electron multipliers (SEM) can be manufactured in flat shapes, for example in the shape of plates as so-called multi-channel plate multipliers. Another type of manufacture uses the pores of a frit consisting of ceramic or glass particles (e.g. microbeads) as secondary electron amplifying channels. In particular, these frit-shaped secondary electron multipliers can also be manufactured with curved surfaces. Their pores are very small; superficially, they look very smooth. With this frit-shaped type of secondary electron multiplier, the dome shapes of the end cap electrode are relatively easy to reproduce.
When the ions impact on the porous surface of this secondary electrode multiplier, secondary electrons are formed in the familiar way and are drawn into the pores by a strong voltage drop in the frit. There they are accelerated by inner fields, form further secondary electrons when they impact on the pore surfaces, and thus form an electron avalanche which emerges on the other side of the frit and can be measured as a real electron current by a collecting electrode. In this way, both positive and negative ions can be measured.
In the case of positive ions, another advantage of the arrangement can be utilized. Those positive ions which impact during enlargement of their oscillation amplitude for ejection onto the end cap electrode opposite the detector end cap electrode also give rise to secondary electrons there. These pass through the ion trap intermediate area in only a few nanoseconds and impact on the secondary electron multiplier. They therefore amplify the ion current measurement by also utilizing those ions which are ejected on the xe2x80x9cwrong sidexe2x80x9d of the ion trap. For this purpose, it is useful to provide the opposite end cap electrode with a surface which has a good yield of secondary electrons.
The method of operating the ion trap according to the invention with a smoothly fitting secondary electron multiplier in the end cap electrode has the additional advantage that it automatically compensates for the lower sensitivity of the secondary electron multiplier for heavier ions. The yield of secondary electrons during the first impact of the ions on the surface decreases greatly as the mass of the ions increasesxe2x80x94at the same ion energy level. Heavy ions, which are generally of particular interest, are thus detected far less sensitively than light ions. However, when operating the detector according to the invention, the impact energy of the ions is automatically increased as their mass increases, since all modern ejection methods operate with a mass-proportional increase of the RF voltage. This voltage determines the impact energy of the ions.